OpenBSD

NetBSD

jakllsch has worked on a partially-complete port of the x86 boot code to the role of native payload. However, with the advent of SeaBIOS, this is likely to become less of a priority. Consult Coreboot and NetBSD for further information.

Currently, GRUB2 refuses to load the kernel due to a small bug in the multiboot header of the kernel, but the kernel still refuses to
work if that is worked around. Maybe they reintroduced some BIOS calls again?

History

The payload was originally intended to be a Linux kernel stored in flash. Flash ROM growth rate was anticipated optimistically however, today there are not many mainboards that actually have enough flash ROM room for a kernel. 512KB can be seen here-and-there and a few boards come with 1MB. Recent kernels really want that MB, and then
you'll only have room for 300-400 KB of initial ramdisk, which could be too small too, depending on the application. During testing, a payload may also be downloaded via X-Modem from the serial debug console, saving flashing time.

So, other payloads are used; the two major ones are FILO and SeaBIOS. Also have a look at GRUB2 and Etherboot (soon to be deprecated in favor of GPXE). FILO loads a kernel from a filesystem on an IDE device and Etherboot loads a kernel from the network or from a filesystem on an IDE device.

If you're using FILO there is no Linux kernel until FILO loads it, and the kernel loaded by FILO (or Etherboot) can absolutely be the one you want to run in your system. Just set it up with the correct root and init commandline so that it can start init.